Add a timeout to JSONHTTPLogHandler

When the server running conftool2git was rebooted, the connection was impossible but not refused, so our audit logger would wait for the connection timeout on the socket, which is set by default in http.client to the general socket timeout.

With this change, we should be able to limit the impact of the server going down to a slight slowdown; if this means we lose some log messages, I think we can live with that.